1. Field of the Invention
The present invention generally relates to the art of microelectronic circuit packaging, and more specifically to a domed lid for an integrated circuit package which transfers a major component of an applied force or pressure from the lid into a sidewall of the circuit package, and resists deflection of the lid into the package.
2. Description of the Related Art
Microelectronic circuits, including hybrid microcircuits, may be encapsulated in a hermetically sealed case or package which provides support and protection. Numerous package configurations are known in the art, those relevant to the present invention including a base, and a continuous sidewall upstanding from the periphery of the base. The sidewall may be integral with the base, or formed separately and welded or brazed to the base. A microcircuit board which includes a plurality of integrated electronic components and interconnecting metallizations is mounted on the base within a cavity defined by the sidewall. Pins extend externally from the cavity through the base for connection of the microcircuit board to an apparatus in which the circuit is incorporated.
The packages are hermetically sealed by a cover or lid which spans the sidewall. The material most widely used for the packages and lids is Kovar, an alloy manufactured by the General Electric Company, including 53% iron, 29% nickel, and 18% cobalt. A gold or nickel plating may be applied to prevent corrosion. The lids are welded or soldered at their peripheries to the upper surface of the sidewall, thereby hermetically sealing the cavity in which the microcircuit board is mounted.
Microcircuit package lids known in the art are either simple flat plates, or have a "hat-shaped" configuration. An example of prior art package lids and sealing methods is presented in an article entitled "SOLDER SEALING OF LARGE COMPLEX HYBRID MICROCIRCUITS", by J. Holley, ISHM National Symposium Proceedings (1969), pp. 5-11. A hat-shaped lid includes a flat central area which is raised from the sidewall of the package by an upstanding edge portion to provide a "T-joint" configuration for welding. Alternatively, a lip, which resembles the brim of a hat, may extend outwardly from the upstanding edge portion.
Prior to shipment, an assembled circuit package must be tested to confirm the hermeticity of the seal. During the test, the package is subjected to substantial pressure, which causes the lid to be bent or deflected downwardly into the cavity. This deflection can cause shorting and/or damage to components, and also induces extreme stress in the weld seals which can cause the seals to fail.
This effect is so pronounced in large packages that central support posts have been provided to limit deflection of the lids into the cavities. However, the support posts cause rework problems in de-lidding and circuit repair, since the lids must be de-welded from the support posts for reworking. The circuit boards also become more sensitive to thermally induced stresses since holes must be formed through their substrates for the posts to contact the bases and lids of the packages. Valuable space is lost on the circuit boards since routing cannot take place in the central areas of the boards through which the holes are formed.